It's no secret that stem cell research is one of the most controversial areas in all of science. Now, recent advances concerning human embryonic stem cells -- and the so-called "14-day rule" -- are threatening to ignite another fundamental debate.

As Julian Huguet explains in this DNews report, most countries permit human embryos to be studied in the lab for up to 14 days after their creation by in vitro fertilization. After that, the cells must be destroyed. As a practical matter, that hasn't been an issue -- researchers haven't been able to grow embryos past that point in the lab, anyway. Until now.

Two separate research teams recently reported growing human embryos 13 and 14 days past fertilization. They halted their studies at that point, but are now calling for a re-assessment of the 14-day rule.

Fourteen days may seem arbitrary, but there's a long history behind the limit.

Originally, the rule was established in a report from the U.S. Department of Health, Education and Welfare back in 1979. A committee of medical doctors, psychologists and theologians arrived at the two-week period, but it was largely a moot point. At the time, embryos could only be kept alive in vitro for a few days.

Subsequent studies reinforced the two-week limit. A 1984 British report cited the "primitive streak," a faint line of cells that appears on embryos around the 14th or 15th day. This streak, it was argued, signifies the moment when the embryo becomes an individual being.

The 14-day rule provided a useful designation, effectively sidestepping debates over personhood and "the soul."

But now that advances have made it possible for embryos to stay alive past the 14-day mark, the debate is likely to flare up again -- and soon -- among scientists, theologians and policymakers.